Heinz makes a "One Carb" ketchup. I don't do ketchup so I don't know if you can find it in your local grocery store, but you can pick it up from netrition here:
http://www25.netrition.com/heinz_one_carb_ketchup.html
It's 1 carb per tablespoon, so you'd have to measure carefully. But if you can't live without it, I think it's a good alternative.